docs: def scrub_output_pre_save(model, **kwargs):
      """scrub output before saving notebooks"""
      # only run on notebooks
      if model['type'] != 'notebook':
          return
      # only run on nbformat v4
      if model['content']['nbformat'] != 4:
          return
  
      for cell in model['content']['cells']:
          if cell['cell_type'] != 'code':
              continue
          cell['outputs'] = []
          cell['execution_count'] = None